The Third Generation Partnership Project (3GPP) has set forth a study item for further enhancements to the uplink; see document RP-122019 (3GPP TSG Meeting #58; Barcelona, Spain; 4-7 Dec. 2012). One objective of this study item is to reduce the uplink control channel overhead for high speed packet access (HSPA) operation; properly reducing control channel overhead can allow more spectrum to be made available for data. For example, document R2-130499 by Vodaphone Group and entitled “Rationale and scenario for HSPA uplink control channel overhead reduction” (3GPP TSG-RAN2 Meeting #81; St. Julian's, Malta; 28 Jan. to 1 Feb. 2013) asserts that the capacity taken up by uplink control channel overhead heavily reduces the uplink data capacity available in the cell.
One type of control signalling in HSPA is the channel quality indicator (CQI), which indicates how much transport block size can be transmitted over the high speed downlink packet access (HSDPA) protocol. Frequent CQI reporting is important to maintain good downlink efficiency because it tells the base station scheduler the data rate that the mobile terminal expects to be able to receive at a given point in time. The CQI value that the mobile terminal (or “user equipment” UE) reports is a combination of the signal to interference ratio (SIR) the UE is experiencing and a function of the multipath environment, the UE receiver type, the ratio of the interference of the UE's own base transceiver station (BTS, also termed a NodeB) as compared with other BTSs, and the expected BTS HSDPA power availability.
Specifically for the HSPA protocol, sub clause 8.2.2 of 3GPP TS 25.308 v11.4.0 describes CQI as being used to select transport format and resource by the high-speed downlink shared channel (HS-DSCH) serving NodeB. For frequency division duplex (FDD) operation, the transmission rate of the measurement report to the network is configured by higher layer signalling. For time division duplex (TDD) operation, a CQI report is associated with each high-speed shared control channel (HS-SCCH) transmission when a UE is operating in the CELL_DCH state.
FIG. 1 shows a signalling diagram showing the relation of CQI reporting on the uplink control channel HS-DPCCH (HS-dedicated physical control channel) and data capacity on the downlink channel HS-PDSCH (HS-physical downlink shared channel). The UE derives CQI based on the measured common pilot channel (CPICH) quality and reports it on the HS-DPCCH to the serving cell that controls the HS-DSCH. The serving cell schedules data to be sent on the HS-PDSCH, using the reported CQI to determine data rate and other transport parameters, and then sends the data as scheduled. Having reported the CQI, the UE can anticipate what transport parameters will be used for that data transmission. Inadequate CQI leads to a downgrade in how efficiently the data can be sent, but excess CQI reporting occupies spectrum that is not available for data.
Reduction of the CQI report has been identified as one way to reduce the uplink control channel overhead for HSPA operation; see for example document R2-130155 by Huawei and HiSilicon entitled “UL control channel overhead reduction for Further EUL Enhancements”, and also R2-130456 by Ericsson and ST Ericsson entitled “CQI report reduction” (both from 3GPP TSG-RAN2 Meeting #81; St. Julian's, Malta; 28 Jan. to 1 Feb. 2013). The former document suggests reducing the CQI reporting by setting the CQI transmission priority to zero when the HS-DSCH transmission is inactive. The latter document suggests examining when it would be appropriate to reduce or discontinue (DTX) the CQI transmission in the uplink and when it is appropriate to have “normal” CQI transmissions. These teachings provide a different approach to reduce CQI transmissions as compared to conventional HSPA practice.